Pierre's really ditching very dated clothes (7)
I believe the answer is:
raiment
'dated clothes' is the definition.
(dated term for clothing)
'pierre's really ditching very' is the wordplay.
'pierre's really' becomes 'vraiment' ('really' in French - Pierre is a French name).
'ditching' is a deletion indicator.
'very' becomes 'v' (abbreviation).
'vraiment' with 'v' taken away is 'RAIMENT'.
(Other definitions for raiment that I've seen before include "Fancy clothes seen on minaret" , "Clothing (literary)" , "Tire man (anag.)" , "Something put on" , "Garment" .)